1. Field of the Invention
The present invention relates to an apparatus and method for low-destructively (low-invasively) measuring mechanical properties within object such as structures, substances, materials, living tissues (liver, prostate, breast, etc.). For instance, measured can be, due to applied stress and/or vibration by arbitrary mechanical sources, generated displacement vector, strain tensor, strain rate tensor, acceleration vector, or velocity vector, etc. within the body. Furthermore, from these measured deformation data, following constants can be measured, elastic constants such as shear modulus, Poisson's ratio, etc., visoc elastic constants such as visco shear modulus, visco Poisson's ratio, etc., delay times or relaxation times relating these elastic constants and visco elastic constants, or density.
On typical applied field, i.e., medical field, such as ultrasonic diagnosis, nuclear magnetic resonance diagnosis, light diagnosis, radio therapeutics, the present method and apparatus can be applied for monitoring tissue degeneration, i.e., treatment effectiveness. Otherwise, on structures, substances, materials, living tissues, measured static and/or dynamic mechanical properties can be utilized for evaluation, examination, diagnosis, etc.
2. Description of a Related Art
For instance, on medical field (liver, prostate, breast, etc.), lesions are proposed to be treated by cryotherapy, or by applying radioactive ray, high intensity focus ultrasound, laser, electromagnetic RF wave, micro wave, etc. In these cases, the treatment effectiveness is proposed to be monitored. Moreover, chemotherapy (anti-cancer drag etc.) effectiveness is also proposed to be monitored. For instance, on radiotherapy etc., the treatment effectiveness can be confirmed by low-invasively measuring degeneration (including temperature change) of the lesion. Otherwise, due to applied stress to the tissue part of interest including lesions, generated deformations and deformation changes are measured, from which the pathological state of the tissue is evaluated such as elastic constants etc. Thus, based on the measured distinct pathological state, the part of interest is diagnosed, or treatment effectiveness is observed.
Temperature is known to have high correlations with elastic constants, visco elastic constants, delay times or relaxation times relating elastic constants and visco elastic constants, and density, etc. Therefore, by measuring the following constants, temperature distribution can be measured, i.e., elastic constants such as shear modulus, Poisson's ratio, etc., visco elastic constants such as visco shear modulus, visco Poisson's ratio, etc., delay times or relaxation times relating these elastic constants and visco elastic constants, or density.
In the past, elastic constants and visco elastic constants have been measured by applying stress at many points and by measuring the responses such as stresses and strains. That is, stress meter and strain meter are used, and sensitivity analysis is numerically performed with utilization of finite difference method or finite element method. Otherwise, in addition to elastic constants, visco elastic constants such as visco shear modulus, visco Poisson's ratio, etc. has been also measured by estimating the shear wave propagation velocity generated by applying vibrations.
As other monitoring techniques, temperature distribution is measured by evaluating nuclear magnetic resonance frequency, electric impedance, ultrasound velocity, etc. However, to measure temperature, these techniques need other relating physical properties of the target tissue. If degeneration occurs on the part of region, the relating physical properties also change; thus resulting severe limitations of the temperature measurement.
Other disadvantage is that the past measurement technique needs many independent deformation fields generated by mechanical sources outside the target body. However, if there exist internal mechanical sources or mechanical sources are uncontrollable, the technique becomes unavailable. That is, the past technique needs all information about mechanical sources, such as positions, force directions, force magnitudes, etc. Moreover, the technique needs stress data and strain data at the target body surface, and needs whole body model (finite difference method or finite element method). Furthermore, low are spatial resolutions of measured elastic constants and visco elastic constants from shear wave velocity.
On the other hand, medical ultrasound diagnosis apparatus can low-invasively image tissue distribution by converting ultrasonic echo signals (echo signals) to image, after transmitting ultrasonic pulses to target tissue and receiving the echo signals at ultrasound transducer. Thus, by ultrasonically measuring tissue displacements generated due to arbitrary mechanical sources, or by measuring generated tissue strains, tissue elastic constants, etc., differences of between lesion and normal tissue can be observed low-invasively. For instance, measured within the body can be, due to applied stress and/or vibration by arbitrary mechanical sources, generated displacement vector, strain tensor, strain rate tensor, acceleration vector, velocity vector, etc. Furthermore, from these measured deformation data, following constants can be measured, elastic constants such as shear modulus, Poisson's ratio, etc., visco elastic constants such as visco shear modulus, visco Poisson's ratio, etc., delay times or relaxation times relating these elastic constants and visco elastic constants, or density.
Then, in the past, tissue displacement has been proposed to be measured to low-invasively diagnose tissue and lesion by evaluating the echo signal changes of more than one time transmitting signal. From the measured displacement distribution, strain distribution is obtained, by which distribution of pathological state of tissue has been proposed to be diagnosed (Japanese Patent Application Publication JP-A-7-55775, JP-A-2001-518342). Specifically, 3 dimensional (3D), 2D, or 1D region of interest (ROI) are set in the target body, and distributions of three, two, or one displacement component are measured, from which in addition to strain tensor distribution, elastic constant distributions, etc. are also numerically obtained.
In addition to ultrasound transducer, as the displacement (strain) sensor, utilized can be known contact or non-contact sensors such as electromagnetic wave (including light) detector etc. As mechanical sources, compressor and vibrator can be, transducer-mounted apparatuses, not transducer-mounted ones, internal heart motion, respiratory motion. If ROI is deformed by ultrasound transmitted from sensor, there may not need other mechanical sources except for the sensor. In addition to the stationary elastic constants, difference of the tissue pathological state includes dynamic changes of elastic constants and temperature due to treatment.
However, as the classical tissue displacement measurement methods assume tissue being deformed only in the axial (beam) direction, when tissue also moves in lateral (scan) direction, the classical method has low axial measurement accuracy. That is, the displacement was determined only by 1D axial processing of ultrasound echo signals (hereafter, echo signal includes rf echo signal, quadrate detection signal, envelop detection signal, and complex signal).
Recently, displacement accuracy is improved by us through development of 2D displacement vector measurement method, i.e., phase gradient estimation method of the 2D echo cross-spectrum based on so-called the 2D cross-correlation processing and the least squares processing. This method can suitably cope with internal, uncontrollable mechanical sources (e.g., heart motion, respiratory motion, blood vessel motion, body motion, etc.).
However, strictly speaking, measurement accuracy of actual 3D tissue displacement becomes low because the method can measure by 2D processing of echo signals two displacement components or by 1D processing one displacement component.
Particularly, as lateral component of echo signal has a narrow bandwidth and has no carrier frequency, lateral displacement measurement accuracy and spatial resolution are much lower compared with axial ones. Thus, the low lateral measurement accuracy degrades the 3D displacement vector measurement and the 3D strain tensor measurement.
Furthermore, when large displacement needs to be handled, before estimating the gradient of the cross-spectrum phase, the phase must be unwrapped, or the displacement must be coarsely estimated by cross-correlation method as multiples of sampling intervals. Thus, measurement process had become complex one.